Aug. 18 (UPI) -- Excitement is building across the country for the biggest celestial event in the contiguous United States since 1979, but experts warn safety first.

All eyes will be pointed to the sky Monday, Aug. 21, when the first total solar eclipse within the contiguous United States since Feb. 26, 1979, will occur. The last eclipse in 1979 passed over the northwestern United States in Washington, Oregon, Idaho, Montana, North Dakota and Canadian provinces.

Every area of the United States, however, will have some portion of the eclipse with 90 percent to 60 percent visible, from the extreme northern border with Canada to the extreme southern border with Mexico.

Experts have been warning people not to look directly at the sun during the eclipse without protective solar filter lenses because doing so can cause severe eye damage, including blindness.

According to NASA, the only safe way to look directly at the sun during the phases of the eclipse, except totality, is through the use of special-purpose solar filters such as eclipse glasses.

The brief period of time when the eclipse is in totality -- when the moon completely covers the sun -- is the only safe time to view the sun without protection. The uneclipsed or partially eclipsed sun can cause significant damage to the eye leading to blindness though.

Only special-purpose solar filter glasses that meet the ISO 12312-2 international safety standard and are labeled as such are safe for eclipse viewing. Specialized solar filters can be purchased to place on cameras, telescopes and binoculars for safe viewing.

Due to the immense interest, many vendors have sold out of the specialized eclipse viewing glasses but the American Astronomical Society has a list of available ISO 12312-2 NASA approved safety glasses.

The society said not all glasses available for purchase meet international safety standards for directly viewing the sun and says that people need to make sure glasses have "ISO 12312-2" printed on them.

Viewers can make their own solar eclipse viewer out of cardboard that shows a projection of the eclipse without viewing it directly. National Geographic also offers tips on how to build a solar eclipse viewer out of cardboard.

Experts caution that, in addition to adults, children should be supervised to make sure they are wearing proper eclipse viewing glasses properly.

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